Preheat oven to 375°F (190°C).

Melt butter in a saucepan over medium heat until foamy.

Sauté onions until they are softened and translucent, about 8 minutes.

Caramelize the onions by continuing to cook them for an additional 20-25 minutes, stirring occasionally, until they are deep golden brown.

Add beef broth to the saucepan and bring to a boil.

Reduce heat to low and simmer for 10 minutes.

Butter slices of bread and toast until crispy.

Assemble the soup by ladling it into oven-proof bowls and topping each bowl with a toasted slice of bread and a sprinkle of cheese.

Bake in the preheated oven for 5-7 minutes, or until the cheese is melted and bubbly.

Serve immediately, garnished with fresh thyme if desired.